Interrogator for detecting adjacent transponders

ABSTRACT

A method for interrogating remote transponders having the steps of: sending an RF interrogation pulse from an interrogator (10), receiving by a first and second transponder (12,12a) the RF interrogation pulse, and establishing in a resonant circuit (130) of each of the transponders (12,12a) an oscillation, the oscillation being established by the coupling of signal energy from the RF interrogation pulse into the resonant circuit (130). After the RF interrogation pulse ends, the first transponder (12) senses the termination of the pulse and initiates a first RF response having a selected duration. A second RF response from the second transponder (12a) will also be detected in the first transponder (12) whose response will be affected by this second RF response. The differing response times in the first transponder (12) for responses affected and unaffected by neighboring transponders may be sensed in the interrogator so the interrogator may detect instances in which the RF responses which it received may have been conflicting.

This application is a continuation-in-part of the followingapplications:

Ser. No. 08/263,904 filed Jun. 22, 1994, now U.S. Pat. No. 5,451,959;which is a continuation of application Ser. No. 07/964,574 filed Oct.21, 1992, now abandoned; which is a continuation of application Ser. No.07/742,134 filed Aug. 8, 1991, now abandoned; which is a division ofapplication Ser. No. 07/655,182 filed Feb. 13, 1991, now U.S. Pat. No.5,053,774; which is a continuation of application Ser. No. 07/216,756filed Jul. 08, 1988, now abandoned and Ser. No. 08/105,538 filed Aug.11, 1993, now U.S. Pat. No. 5,457,461.

CROSS-REFERENCE TO RELATED PATENTS

The following coassigned patents are hereby incorporated herein byreference:

    ______________________________________                                        U.S. Pat No.   Filing Date TI Case No.                                        ______________________________________                                        5,053,774      2/13/91     TI-12797A                                          07/981,635     11/25/92    TI-16688                                           ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to a method for detecting adjacenttransponders.

BACKGROUND OF THE INVENTION

There is a great need for devices or apparatuses which make it possibleto identify or detect as regards their presence at a predeterminedlocation objects which are provided with such devices or apparatuses incontactless manner and over a certain distance. An additional needexists to be able to determine if two or more transponders are adjacentto each other.

SUMMARY OF THE INVENTION

The preferred embodiment of the present invention detects the presenceof adjacent transponders by measuring the duration of the responsesignal from the transponders. In the embodiments described in U.S. Pat.No. 5,053,774 and patent application Ser.No. 07/981,635, both assignedto Texas Instruments by Josef H. Schuermann, the normal period forresponse to an RF interrogation is approximately 20 ms during which thetransponder is normally operable to provide a 128 bit response message.After this 20 ms time period, the output of the transponder issquelched. The squelching of the response signal is accomplished by atransponder timer, which is triggered by the end-of-burst (when theexciter signal has ceased). The timer counts up to 128 (bits) and thendischarges the transponder charge capacitor.

The present invention is the first to utilize the coupling betweenadjacent transponders in order to determine whether two or moretransponders are adjacent. When two transponders are adjacent, they bothare charged up and respond individually to an RF interrogation. Thus,when one transponder sends a response signal, its field strengthradiates into the other transponder. This radiated signal will interferewith the oscillation that is being maintained in the other transponder'sresonant circuit. The interference from the cross-coupled responses willcause a beat in the resonant circuit of each of the transponders. This"beat" is analogous to the phenomenon heard by musicians when tuning twoinstruments that are out of tune. Because the instruments have differentfrequencies, a time-variant pattern of constructive interference (wherethe acoustic signals are in phase and increase the sound intensity) anddestructive interference (where the acoustic signals are 180 degrees outof phase and decrease the sound intensity) can be heard. Thistime-variant pattern is referred to as a "beat."

This beat effect can occur in both transponders. Periods of destructiveinterference simulates for either transponder the end-of-burst effect,causing the timer to repeatedly reset (because any end-of-burst willreset the timer). Resetting the timer opens a new 128 msec window forthe logic and transponder to transmit a data signal. Therefore thetransponder's discharge function is repeatedly disabled by this beateffect and the transponder transmits until the charge capacitor is fullydischarged.

A beat effect can also occur between the preferred embodimenttransponder and other transponders, such as those made by manufacturersother than the assignee of the current invention. Interference fromanother type of transponder will still cause a beat in the resonantcircuit and repeated resetting of the timer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block circuit diagram of a preferred interrogator and baseunit;

FIG. 2 is a block circuit diagram of a preferred transponder;

FIG. 3 is a more general block diagram of the preferred arrangementshowing its most salient features; and

FIG. 4 shows signal waveform graphs for antenna signals for a normaltransponder response with no interfering adjacent transponders and for atransponder response with another transponder located in closeproximity.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In reference to FIG. 1, the transponder arrangement to be describedincludes an interrogator 10 and a transponder 12. The interrogator 10 ispreferably constructed to be held in the hand of an operator and totransmit a RF interrogation pulse on actuation of a key 14. Thisinterrogator 10 also has the capacity of receiving RF signals anddetecting information contained in the signals. The RF signals come fromthe transponder 12 which according to this embodiment replies to thetransmission of a RF interrogation pulse by sending back the RF signalhaving preferably the same frequency as the interrogation pulse.Preferably the RF signal is modulated with data by the transponder 12using frequency shift keying (FSK) modulation. Associated with theinterrogator 10 is a base unit 16 which is constructed as a stationaryunit. The functions of the interrogator 10, the transponder 12 and baseunit 16 and their interaction will be described in more detailhereinafter. Firstly, the makeup of these units will be explained.

The interrogator 10 contains as central control unit a microprocessor 18which is responsible for the control of the function sequences. A RFoscillator 20 generates RF oscillations as soon as it has been set inoperation by a signal at the output 22 of the microprocessor 18. Theoutput signal of the RF oscillator 20 can be supplied either via aswitch 24 and an amplifier 26 or via a switch 28 and an amplifier 30 toa coupling coil 32. The switches 24 and 28 are controlled by themicroprocessor with the aid of signals emitted at its outputs 34 and 36respectively. Coupled to the coupling coil 32 is a coil 38 of a resonantcircuit which consists of the coil 38 and the capacitor 40. In serieswith the coil 38 and the capacitor 40 is a resistor 44 bridgeable by aswitch 42 and a further switch 46 lies between the resistor 44 andground. The switches 42 and 46 are controlled by the microprocessorwhich emits at its outputs 48 and 50 corresponding control signals. Whenthe switch 46 is closed the resonant circuit comprising the coil 38 andcapacitor 40 acts as parallel resonant circuit whilst when the switch 46is open it acts as series resonant circuit. The coil 38 acts astransmitting and receiving coil which transmits the RF interrogationpulse supplied to it by the oscillator 20 and receives the RF signalsent back by the transponder 12.

The RF signals received by the resonant circuit are supplied to twoamplifiers 52, 54 which are so designed that they amplify the RF signalsreceived and limit them for pulse shaping. Connected to the amplifiersis a parallel resonant circuit 56 which ensures the necessary receptionselectivity. The output of the amplifier 54 is connected to a clockgenerator 58 which from the signal supplied thereto generates a clocksignal and supplies the latter to the input 60 of the microprocessor 18.

In addition, the output signal of the amplifier 54 is supplied to ademodulator 62 which demodulates the signal applied thereto and suppliesit to the input 64 of the microprocessor 18.

The information contained in the received RF signal is supplied afterthe demodulation in the demodulator 62 via the microprocessor 18 to arandom access memory 66 so that it can be stored in the latter. Betweenthe microprocessor 18 and the random access memory 66, a bi-directionalconnection 68 is disposed which makes it possible to enter informationfrom the microprocessor 18 into the random access memory 66 and also totransfer information in the opposite direction. The information storedin the random access memory 66 can be taken off at a jack 70.

A display unit 72 fed by the microprocessor 18 makes it possible for theoperator to view the data contained in the RF signal received.

Since the interrogator 10 is a portable device, a rechargeable battery74 is provided as a power supply. The output voltage of the battery 74is supplied after closing a switch 76 to the terminals designated by "+"of selected chips in the interrogator 10. The supply voltage is howeversupplied to the two amplifiers 52, 54, the clock generator 58 and thedemodulator 62 via a separate switch 78 which is controlled by themicroprocessor 18. This makes it possible for those circuit elements tobe supplied with voltage and thus active only during a predeterminedperiod of time within the total operating cycle.

The battery 74 can be charged by a voltage induced in a coil 80,rectified in a rectifier 82 and smoothed by means of a capacitor 84.Preferably, the voltage is induced in coil 80 via a coil 112 in the baseunit 16. A charge sensor 86 detects when a charge voltage is induced inthe coil 80, i.e. a charging operation of the battery 74 is takingplace. It then emits to the input 88 of the microprocessor 18 acorresponding message signal.

A further switch 90, controlled by means of a signal from the output 92of the microprocessor 18, can in the closed state supply the outputsignals of the RF oscillator 20 via an amplifier 94 to a coupling coil96. The switch 90 is typically used to activate the sending of a RFinterrogation pulse to a transponder 12 to initiate a data transfer toor from the transponder 12.

With the aid of a modulator 98 the RF oscillator 20 can be modulated.The modulation signal necessary for this purpose is supplied to themodulator 98 by the microprocessor 18 via a switch 100 which iscontrolled by means of a signal from the output 102 of themicroprocessor. The modulation signal from the microprocessor 18 issupplied when the switch 100 is closed also to a coupling coil 104.

The base unit 16 also illustrated in FIG. 1 is a stationary unit whichis connected via a jack 106 to the mains supply network. In a powersupply 108 the operating voltage for a charging voltage generator 110 isgenerated, the output signal of which is supplied to a coil 112. Aswitch 114 is inserted between the power supply 108 and the chargevoltage generator 110. The switch 114 is closed whenever theinterrogator 10 is placed on the base unit 16. This is shown in FIG. 1symbolically by a sort of actuating button 116 at the boundary line ofthe interrogator 10. The coils 112 and 80 are arranged in the base unitand interrogator 10 spatially in such a manner that they cooperate likethe primary winding and secondary winding of a transformer when theinterrogator 10 is placed on the base unit 16. In this manner thebattery 74 can be charged contactless as often as required. The coils 96and 104 in the interrogator 10 are so arranged that they are spatiallyvery close to a coil 118 when the interrogator 10 is placed on the baseunit 16. In this manner a contactless signal transmission between thecoil 96 and the coil 104 on the one hand and the coil 118 on the otheris possible. A demodulator 120 serves to demodulate the signals comingfrom the coil 118.

The preferred embodiment transponder 12 illustrated in FIG. 2 containsfor reception of the RF interrogation pulse a parallel resonant circuit130 having a coil 132 and a capacitor 134. Connected to the parallelresonant circuit 130 is a capacitor 136 serving as energy accumulator.In addition the parallel resonant circuit 130 is connected to a RF bus138. The resonant circuit 130 acts as a receiver and as a transmitter asis well known in the art. A clock regenerator circuit receives the RFsignal from the RF bus 138 and regenerates a clock signal 139 having asubstantially square waveform. An end of burst detector 142 connected tothe RF bus 138 has the function of monitoring the power level of a RFcarrier at the RF bus 138. Such a RF carrier occurs at the RF bus 138whenever the parallel resonant circuit 130 receives a RF interrogationpulse from the interrogator 10. The end of burst detector 142 emits atits output a RF threshold signal of predetermined value as soon as thepower level of the RF carrier at the RF bus 138 drops below apredetermined threshold value. By connecting a diode 144 to the RF bus138, the RF carrier is rectified and as a result the capacitor 136 ischarged. The energy stored in capacitor 136 is proportional to theenergy contained in the RF interrogation pulse. Thus, after reception ofthe RF interrogation pulse a DC voltage can be taken off at thecapacitor 136. A zener diode function 146 connected to the capacitor 136ensures that the DC voltage which can be tapped off does not exceed avalue defined by the zener voltage of the diode 146 in practicalimplementations such as within an integrated circuit, the zener diodefunction 146 might be accomplished by a number of circuits well known inthe art for limiting voltage. A zener diode function 146 serves asimilar function to prevent the voltage on the RF bus 138 from becomingtoo great. Initially upon interrogation of the transponder 12 theinterrogator 10 sends a RF signal to the transponder for the expresspurpose of charging the transponder 12. This is referred to as thecharge phase.

A Power-On-Reset (POR, not shown) circuit provides a POR signal to astart detect circuit 154. This POR circuit monitors the Vcc level and isactivated when the Vcc level rises from a level below a certain DCthreshold to a level above a certain DC threshold. Typically, the PORsignal occurs within the charge phase of the transponder. POR circuitsare well known in the art, indeed they are commonly used in almost allof the class of circuits known as "state machines" so that the circuitsmay be initialized to a known state. The start detect circuit 154 uponreceiving the POR signal will then monitor the output 150 of end ofburst detection circuit 142. At output 150 is provided an end of burstsignal (EOB). Upon receipt of an affirmatively stated EOB subsequent tothe affirmatively stated Power-On-Reset signal, start detect circuit 154switches power to the clock regenerator circuit 140 via switch 156.Clock regenerator circuit 140 preferably will clean up the signal fromresonant circuit 130 and provide a regenerated RF clock which ispreferably a square wave. Output of start detect circuit 154 will remainpositively asserted until a subsequent POR is received. All parts of thetransponder other than the clock regenerator 140 are continuouslysupplied with Vcc, but preferably consume a negligible amount of powerin their inactive states (i.e. when the clock regenerator 140 isinactive) due to the utilization of low power CMOS technology.

With further reference to FIG. 2, a divider 158 receives clock signal139 and divides its frequency, preferably by a factor of eight. A pluckcircuit 192 preferably sends a momentary pulse each time it is sotriggered by the divided clock signal as received from divider 158. Thispluck circuit 192 maintains the oscillation of resonant circuit 130 bymomentarily rendering field-effect transistor or FET 190 conductive andforming therethrough a conductive path between the resonant circuit 130and ground via RF bus 138 such that the resonant circuit gainselectrical energy from storage capacitor 136. This pluck circuit 192 isfiguratively named to describe the maintaining of the oscillation ofresonant circuit 130 much like the plucking of a guitar string maintainsthe oscillation of the guitar string. While this "plucking" action willmomentarily lower the voltage on RF bus 138 the duration of the pulse isnot sufficient given the channel resistance of FET 190 to lower thevoltage below the threshold to trigger the end of burst detectioncircuit 142 to activate. A second divider 160 divides the clock signal139 again by a factor of two such that the clock frequency at the outputof divider 160 is 1/16 the original clock frequency.

Still referring to FIG. 2, the read circuitry for the preferredembodiment will now be described. Connected to the output of seconddivider 160 is the shift clock input 176 of an output shift register 172such that data is shifted through this register 172 at 1/16 the originalclock frequency. Output shift register 172 receives a parallel load frommemory 168 or another source via data bus 220 upon receipt at itsshift/load₋₋ input 174 of a "load_(--") signal from the output of startdetect circuit 154. Subsequent to loading of the output shift register172 the signal from start detect circuit 154 is asserted positively andhence a "shift" signal is received at the shift/load₋₋ input 174 ofregister 172. While the "shift" signal is positively asserted data willbe shifted through output shift register at 1/16 the original clockfrequency due to the clock signal received at the shift clock input 176.As shown in the figure, the data recirculates through the output shiftregister 172 via the data path 182 and also feeds the gate of FET ormodulator 200 via data path 182. The output shift register data arepreferably low for a certain time (Pre-Bit-Time) and high or lowdependent upon the data loaded therein. The Pre-Bit-Time is used in thisembodiment to allow the interrogator receiver coil 38 time to recoverfrom power burst overload (Charge Phase) and it is used to discriminatethe read function from the write function as will be described below.While the output of output shift register 172 is low then the FET 200 isnot conducting. While the output of output shift register 172 is highFET 200 conducts, thereby connecting capacitor 198 to the resonantcircuit 130 and lowering the resonant frequency thereof. Essentially,the FET 200 acts as a switch under control of output shift register 172,connecting or leaving unconnected capacitor 198, which modulates thefrequency of the resonant circuit 130. In this manner a frequencymodulation of the resonant or carrier frequency of resonant circuit 130is accomplished in response to the data applied to FET 200. A low or"zero" signal is represented when the original resonant frequency of theresonant circuit 130 is maintained throughout a bit period. A high or"one" signal is represented when the new resonant frequency of theparallel combination of original resonant circuit 130 in parallel withcapacitor 198 occurs within a bit period.

With still further reference to FIG. 2, the operation of a dischargelogic circuit will now be described. A timer 184 receives at its input186 the output of divider 160 and divides the clock signal by yetanother factor of 128. The division factor of timer 184 is 128 in thisinstance due to the preferred data transmission bit length of 128. Ifthis bit length were to change, then the division factor of timer 184would preferably change correspondingly. A diode 210 maintainsunidirectional current flow from the timer 184 into a parallel RCcombination of a capacitor 212 and a resistor 214 which maintain chargeon the gate of a field-effect transistor or FET 216 for a knownduration. With the diode 210 the capacitor 212 may be charged by timer184 but must be discharged through resistor 214. The FET 216 serves toprovide, when the gate of FET 216 maintained above a threshold voltageby the parallel combination of resistor 214 and capacitor 216, a lowimpedance discharge path for storage capacitor 136 to ground. In thismanner, after transmission of a complete data frame in this case of 128bits (read phase) from the transponder 12 to the interrogator 10, theremaining energy in the transponder 12 is eliminated by a shod circuitacross the charge capacitor 136. This action secures that thetransponder is correctly initiated during the next charge phase and doesnot rest in a undefined or incorrect state such that a subsequentcharge-up could be blocked. Additionally by this function, eachtransponder 12 within the field of the interrogator 10 has an identicalstart condition.

Still referring to the circuitry of transponder 12 as shown in FIG. 2,the circuitry with which data may be written into transponder 12 (thewrite function) will now be described. In a preferred embodiment of theinvention the interrogator 10 may pulse pause modulate (PPM) the RFinterrogation pulse as shown in FIG. 4. This signal is reflected on RFbus 138. As is well known in the art, a pulse pause modulation systemoperates by alternatively activating and deactivating a carrier wave.During the time period in which the carrier is deactivated, the end ofburst detector 142 senses a decrease in RF energy and is activated.After the start detect circuit 154 is enabled by the POR signal, thestart detect circuit 154 is then activated by the first EOB signalcaused by the Start Bit (see FIG. 4). Although a Start Bit is used inthis preferred embodiment due to the fact that each data bit status istransmitted by the presence or absence of a carrier wave off phase aswill be further described. Other embodiments are possible, however, inwhich no Start Bit needs to be transmitted. The duration of the timeperiod in which the carrier is deactivated, known as the "off" phase, isshorter than the Pre-Bit-Time of the read phase. This particularrequirement is used within this embodiment due to the fact that duringthe off phase, the output shift register 172 will start to shift. Butsince the Pre-Bit-Time is greater than the off phase the output shiftregister is unsuccessful in shifting anything but zeros out, so in factFET 198 is never activated and inverted and no unwanted modulation ofthe carrier 138 will occur. The EOB signal is then deactivated when thecarrier returns. The activations and deactivations of the EOB signalserve to provide a data stream to the data input 226 input of inputshift register 228. Regardless of the transitions of EOB start detectcircuit 154 maintains its output active until a new POR signal isreceived, thus maintaining power to clock regenerator circuit 140 viaswitch 156.

A fourth divider 162 is provided receive the clock signal from seconddivider 160 and the divide clock signal again by 16 in order to supplythe clock input 227 of the input shift register 228 with an Input Clocksignal. In the preferred embodiment, then, the write data rate is 1/256of the resonant frequency or receive clock frequency. Provision must bemade that the data is shifted into the input shift register 228 whilethe data is stable. This can be assured in the following manner. Thefourth divider 162 is activated by the start detect circuit 154 via ANDgate 155. Each consecutive "zero" bit or "low" bit received by end ofburst detection circuit 142 positively asserts the output 150 of theburst detection circuit. The positively asserted signal is then receivedthe negative logic input of AND gate 155. The negative logic input isindicated by a "bubble" at the input of AND gate 155 as is well known inthe art. By definition of the AND function the output of AND gate 155will then be negatively asserted, thereby clearing the fourth divider162 and synchronizing the Input Clock to the Input Data.

End detection circuit 234 detects the end of a data frame if a certainbit combination is in the input shift register 228 and thereuponactivates the programming logic 232 if a programming command has beenpreviously received by command decoder 230. The data is then transferredfrom input shift register 228 to memory 168 or another memory viaparallel data bus 220. Preferably, the memory to which the data istransferred is an electrically-erasable programmable read only memory(EEPROM).

End of burst detector 142 is generally acting in the capacity of a pulsepause modulation (PPM) demodulator. Many other modulations schemes areknown in the art for radio communication and instead of the end of burstdetector, another demodulator might be used for another of such schemes.

Provision is also made in this preferred embodiment to initiate a testsequence via test logic 236. Test logic 236 receives signals fromcommand decoder 230 and data from data bus 220 and may initiate numeroustest routines such as are commonly implemented in the field of logiccircuit design. The results of these test routines may be placed on databus 220 and output by shift register 172 to the modulation circuitry viafield-effect transistor 200.

A programmable tuning network 238 is provided in the preferredembodiment of the present invention. This programmable tuning network238 operates by switching a network of parallel capacitors 240, eachcapacitor 240 being connected through a field-effect transistor or FET242 to ground. Each field-effect transistor is connected to a latch 244which receives and latches data from the memory 168 or from commanddecoder 230 via data bus 220 under control of a latch signal 246 fromthe command decoder 230. By switching a field-effect transistor 242 to aconducting "ON" state, its associated capacitor 240 is connected inparallel with parallel resonant circuit 130. This added capacitance willlower the resonant frequency of the parallel resonant circuit 130. Byswitching a field-effect transistor 242 to a non-conducting "OFF" state,its associated capacitor 240 is floating and has no effect on theparallel resonant circuit 130. A network 238 of FET/capacitor pairs240,242 can provide many different values of added capacitancesdepending on the combinations of each capacitor's 240 relative value asis well known in the art. Alternatively, latch 244 could be aone-time-programmable (OTP) memory such that the data is fixedly storedtherein and the device may be permanently programmed to set the value ofprogrammable tuning network 238.

The factors of the dividers disclosed in the embodiments listed hereinare chosen to fit the particulars of each design. The division factorsshould be selected in each case to most optimally perform the task forwhich they were designed.

With reference to FIG. 3 in conjunction with FIGS. 1-2, it will beassumed that within the range of the coil 38 transmitting the RFinterrogation pulse at least two transponders 12,12a are present.Power/Xmit circuitry 300 is provided to drive the coil 38. As analternative configuration, a separate antenna 39 might be provided asthe "send" antenna, while the coil 38 could be used as the "receive"antenna 38. Preferably however, coil 38 would act as both "send" and"receive" antenna. Power/xmit circuitry 300 might preferably comprisethe oscillator 20 and the amplifier 26 of the preferred embodimentinterrogator 10 of FIG. 1. The coil 132 of the parallel resonant circuit130 (FIG. 2) of the transponder 12,12a then receives this RFinterrogation pulse so that the resonant circuit 130 is stimulated tooscillate. By means of the rectifier diode 144 the RF oscillation at theportion 138a of the RF bus 138 is rectified and the capacitor 136 ischarged with direct current. For the energy supply of transponders12,12a thereafter, only the energy stored in the capacitor 136 is used.A "read" circuit 302 is then provided to receive the transponderresponse. The "read" circuit 302 preferably comprises the clockgenerator 58 which could also provide a carrier detect (CD) signal tothe microprocessor, or the microprocessor 18 might interpret the absenceor presence of a clock signal to be the carrier detect. A timer 304would preferably be provided within the microprocessor 18 of theinterrogator 10. The timer 304 would then measure the duration of thepresence of a clock signal. Given the data signal from the "read"circuit 302 and the timer 304 output, it would be possible for thedecoder 306 to determine if: a) a good read occurred in which the datawas valid and CD ≦20 ms; b) no read occurred such that CD was notdetected; and c) CD ≧20 ms, therefore more than two transponders arepresent in proximity to the interrogator. The functions of decoder 306preferably reside within microprocessor 18 of the preferred interrogator10.

With reference now to FIG. 4, with the termination of the reception ofthe RF interrogation pulse, the RF level at the RF bus 138 begins todrop. The end-of-burst detector 142 detects this decrease and at itsoutput 150 emits a EOB signal of predetermined value as soon as thepower level drops below a predetermined level. The timer 184 is reset byEOB.

After termination of the excitation pulse at the output of the pluckcircuit 192 the field-effect transistor 190 is rendered nonconductive sothat no more current can flow through the coil 132. Since however theparallel resonant circuit 130 has a high quality, the RF carrieroscillation does not cease immediately but the resonant circuitcontinues to oscillate with a damped oscillation. The divider 158,dividing the frequency of the RF carrier wave by two, emits after thesecond oscillation period a signal to the monoflop 192 which is therebytriggered. The monoflop 192, for the duration of its hold time, appliesa maintenance pulse to the field-effect transistor 190. By themaintenance pulse from the pluck circuit 192, for the duration of thehold time, a current flow is caused through the coil 132 and this meansthat for a short time energy is "pumped" into the RF carrier wavegenerator. This pumping effect occurs after every nth oscillation periodof the RF carrier wave, due to the use of the divider 158.

It will be assumed that data are fixedly stored in the memory 168 andthe data are uniquely assigned to the transponders 12,12a. The data mayfor example consist of 128 bits. With the timing of the clock signalsapplied to the clock input 176 the information present in the memory 168is transferred to the output shift register 172. For this transferoperation, 128 pulses are required because then all the 128 data bits ofthe information are contained in the shift register 172. By alsocounting the clock pulses, the timer 184 determines when the transfer ofthe data is completed. After receiving 128 pulses, the timer 184 emitsat its output 188 a signal to the FET 216 through diode 210. Thedivision factor of timer 184 is 128 in this instance due to thepreferred data transmission bit length of 128. If this bit length wereto change, then the division factor of third timer 184 would preferablychange correspondingly. A diode 210 maintains unidirectional currentflow from the timer 184 into a parallel RC combination of a capacitor212 and a resistor 214 which maintain charge on the gate of afield-effect transistor or FET 216 for a known duration. With the diode210 the capacitor 212 may be charged by timer 184, but must bedischarged through resistor 214. The FET 216 serves to provide, when thegate of FET 216 maintained above a threshold voltage by the parallelcombination of resistor 214 and capacitor 216, a low impedance dischargepath for storage capacitor 136 to ground. In this manner, aftertransmission of a complete data frame in this case of 128 bits (readphase) from the transponder 12 to the interrogator 10, the remainingenergy in the transponder 12 is normally eliminated by a short circuitacross the charge capacitor 136. This action secures that thetransponder is correctly initiated during the next charge phase and doesnot rest in a undefined or incorrect state such that a subsequentcharge-up could be blocked.

In the instance when two or more transponders 12,12a lie adjacently, itis desirable for the interrogator 10 to detect such a condition to avoidconsidering the interfering signals from the transponder 12,12a as validdata. By making use of the consequence of cross-coupling betweenadjacent transponders 12,12a the interrogator 10 can sense the presenceof adjacent transponders 12,12a. If such a condition is detected, theinterrogator 10 can then take such action as a more rigorous orselective addressing of the transponders 12,12a or notification of theuser. For the case with the two adjacent transponders 12,12a, both arecharged up and respond individually so as one transponder 12 answers,its field strength radiates into the other transponder 12a because ofthe close coupling between them. This close coupling will cause a "beat"in each of the transponder's resonant circuits 130 because theirfrequencies differ slightly. This "beat" during periods of destructiveinterference (e.g. the signal in the resonant circuit 130 of onetransponder 12 perhaps 180 degrees out of phase with the coupled signalfrom the other transponder 12a) will cause the end-of-burst detector 142to reset the timer 184. Resetting the timer 184 opens a new 128 bitwindow for the response. This will continuously happen in eachtransponder 12,12a. Thus, instead of the signal oscillation energy inthe resonant circuit 130 being damped after 20 ms, the naturaloscillation is allowed to die out with the normal parasitic dampingeffects. In practical examples, this new time period is approximately 40ms.

It can be further determined if there is another type of transponder,that is to say, not the type of transponder described herein as thepreferred embodiment, adjacent to the preferred transponder 12. In sucha case, because the other transponders have more losses (e.g. operatewith lower Q-factors and consume more energy during the function fromthe preferred transponder 12), the transponder 12 delivers more energyfrom its own oscillation to the adjacent transponder and is fasterdischarged. Thus, in a practical example, the transponder may nowdischarge in about 30 ms instead of 40 ms. In either case, the dischargeperiod is greater than the 20 ms, had the oscillation been damped as fora normal interrogation cycle.

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims.

For example, "microcomputer" is used in some contexts to mean thatmicrocomputer requires a memory and "microprocessor" does not. The usageherein is that these terms can also be synonymous and refer toequivalent things. The phrase "processing circuitry" or "controlcircuitry" comprehends ASICs (application Ser.specific integratedcircuits), PAL (programmable array logic), PLAs (programmable logicarrays), decoders, memories, non-software based processors, or othercircuitry, or digital computers including microprocessors andmicrocomputers of any architecture, or combinations thereof. Memorydevices include SRAM (static random access memory), DRAM (dynamic randomaccess memory), pseudo-static RAM, latches, EEPROM(electrically-erasableprogrammable read-only memory), EPROM (erasable programmable read-onlymemory), registers, or any other memory device known in the art. Wordsof inclusion are to be interpreted as nonexhaustive in considering thescope of the invention.

Implementation is contemplated in full-duplex transponder arrangementsor half-duplex transponder arrangements. Frequency shift keying (FSK)modulation is envisioned as a possible data modulation scheme, as wellas pulse-pause modulation, amplitude shift keying (ASK), quadrature AM(QAM) modulation, quadrature phase shift keying (QPSK), or any othermodulation. Different types of multiplexing such as time or frequencymodulation might be effected to avoid cross-signal interference.Implementation is contemplated in discrete components or fullyintegrated circuits in silicon, gallium arsenide, or other electronicmaterials families, as well as in optical-based or othertechnology-based forms and embodiments. It should be understood thatvarious embodiments of the invention can employ or be embodied inhardware, software or microcoded firmware.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. An interrogator for wireless communication withat least two transponders, said interrogator comprising:a. an RFoscillator; b. a transmitter that receives the output of said RFoscillator and initiates an RF interrogation signal for interrogatingsaid at least two transponders; c. an antenna operable to receive theoutput of said transmitter and transmits said RF interrogation signal tosaid at least two transponders, thereby causing each of saidtransponders to transmit RF responses, and operable to receive said RFresponses; d. a receiver for receiving said RF responses via saidantenna upon termination of said RF interrogation pulse; and e. acontrol circuit, said control circuit operable to initiate said RFinterrogation signal in said transmitter, to disable the output of saidtransmitter and thereby enable reception of at least one of said RFresponses upon termination of said RF interrogation signal, to receiveat least one of said RF responses from said receiver, and to measure theduration of said at least one RF response thereby determining whethermultiple transponders exist adjacent to each other.
 2. The interrogatoraccording to claim 1 wherein said antenna is a coil.
 3. The interrogatoraccording to claim 2 wherein said interrogator further comprises anotherswitch operable under control of said control circuit to connect acapacitor in parallel with said coil to form a parallel resonant circuitelectrically coupled to said transmitter, said switch alternativelyoperable under control of said control circuit to connect said capacitorin series with said parallel resonant circuit, thus interposing saidcapacitor between said coil and said receiver.
 4. The interrogator ofclaim 1 wherein the duration of said RF response varies due todestructive interference between an oscillation established in aresonant circuit of one of said at least two transponders and the otherof said at least two transponders.
 5. The interrogator of claim 1wherein said control circuit is further operable to initiate selectiveaddressing of said at least two transponders in such situations wheresuch transponders are determined to be adjacent to each other.
 6. Theinterrogator of claim 1 wherein said interrogator further comprises adisplay unit under control of said control circuit and wherein saidcontrol circuit notifies the user of said interrogator via said displayunit of a condition in which transponders are determined to be adjacentto each other.
 7. An interrogator for wireless communication with atleast two transponders, said interrogator comprising:a. an RFoscillator; b. a "send" antenna for transmitting an RF interrogationsignal to said at least two transponders c. a "receive" antenna forreceiving RF responses from each of said at least two transponders; d. atransmitter that receives the output of said RF oscillator and initiatesan RF interrogation signal for interrogating said at least twotransponders; e. a receiver for receiving said RF responses via saidantenna upon termination of said RF interrogation pulse; f. a timer inelectrical communication with said "receive" antenna, said timeroperable to measure the duration of at least one of said RF responses;and g. a control circuit, said control circuit operable to initiate saidRF interrogation signal in said transmitter, to disable the output ofsaid transmitter and thereby enable reception of at least one of said RFresponses upon termination of said RF interrogation signal, and toreceive at least one of said RF responses from said receiver, and toreceive from said timer the result of its measurement of the duration ofsaid at least one of said RF responses, thereby determining whethermultiple transponders exist adjacent to each other.
 8. The interrogatorof claim 7 wherein said control circuit is further operable to initiateselective addressing of said at least two transponders in suchsituations where such transponders are determined to be adjacent to eachother.
 9. An interrogator for communication with at least onetransponder, said interrogator comprising:a. a control circuit; b. an RFoscillator for generating a carrier wave; c. a modulator for modulationof said carrier wave; d. a transmitter which receives the output of saidmodulator and transmits at least one RF programming sequence havingwrite data modulated thereon for programming said transponder and atleast one RF interrogation pulse of a first frequency for interrogatingsaid transponder thereby causing said transponder to return read datastored therein in the form of a RF response of a second frequency; e. aswitch for disabling the output of said transmitter and enablingreception of said RF response upon termination of said RF interrogationsignal; f. a receiver for receiving said RF response upon termination ofsaid RF interrogation pulse; and g. a timer in electrical communicationwith said receiver, said timer operable to measure the duration of atleast one of said RF response and to notify the control circuit of saidmeasurement whereby said control circuit can determine whether multipletransponders are located within range of said interrogator; and h. ademodulator for demodulation of said read data from said RF responseunder control of said control circuit, said control circuit onlydemodulating said RF response in the situation where the timer hasdetermined that only a single transponder has responded to said RFinterrogation.